Certain image forming devices use an electrophotographic imaging process to develop toner images on a media sheet. The electrophotographic process uses electrostatic voltage differentials to promote the transfer of toner from component to component. For example, a voltage vector may exist between a developer roll and a latent image on a photoconductive element. This voltage vector helps promote the transfer of toner from the developer roll to the latent image in a process that is sometimes called “developing the image.” A separate voltage vector may exist between the photoconductive element and a transfer member to promote the transfer of a developed image onto a substrate. In each instance, the toner transfer occurs in part because the toner itself is charged and is attracted to surfaces having an opposite charge or a lower potential.
The effective transfer of toner within an image forming device is usually dependent on many variables, including environmental conditions such as temperature and humidity. For example, in some systems there is an inverse relationship between humidity and transfer member resistance. Some image forming devices use dedicated temperature and humidity sensors to detect environmental conditions. These devices may alter operating parameters, such as the transfer bias applied to a transfer member, in response to the detected environmental conditions.
Other image forming devices measure the voltage-current characteristics of a test signal propagated through components within the device. For example, some image forming devices transmit a signal through the interface between a transfer member and a photoconductive member. The resistance and capacitance characteristics over this interface change in relation to environmental conditions. Thus, the measured resistance/capacitance characteristics may be mapped in memory to environmental values or to actual operating parameters. Accordingly, device operating parameters may be set in response to the detected resistance/capacitance values.
Unfortunately, the mapped resistance/capacitance values may not account for component deterioration that occurs with wear and use. Over time, the correlation between the mapped resistance/capacitance values and suitable operating parameters may change. For example, the photoconductive layer capacitance increases with wear, thereby reducing the effective resistance. Thus, unless age and wear are accounted for, the device operating parameters that are set in response to detected resistance/capacitance values may produce degraded images.